From f806ad6bf0fad5a720c9b87310d86838612f3b7a Mon Sep 17 00:00:00 2001
From: FintanH
Date: Wed, 31 Jul 2019 12:15:12 +0100
Subject: Reuse the merge_maps function to implement the right-biased union

---
 dhall/src/phase/normalize.rs |  2 +-
 dhall/src/phase/typecheck.rs | 12 ++++++------
 2 files changed, 7 insertions(+), 7 deletions(-)

(limited to 'dhall/src')

diff --git a/dhall/src/phase/normalize.rs b/dhall/src/phase/normalize.rs
index be2ba51..da19dc1 100644
--- a/dhall/src/phase/normalize.rs
+++ b/dhall/src/phase/normalize.rs
@@ -372,7 +372,7 @@ enum Ret<'a> {
     Expr(ExprF<Thunk, X>),
 }
 
-fn merge_maps<K, V>(
+pub(crate) fn merge_maps<K, V>(
     map1: &HashMap<K, V>,
     map2: &HashMap<K, V>,
     mut f: impl FnMut(&V, &V) -> V,
diff --git a/dhall/src/phase/typecheck.rs b/dhall/src/phase/typecheck.rs
index 2315edb..9107e36 100644
--- a/dhall/src/phase/typecheck.rs
+++ b/dhall/src/phase/typecheck.rs
@@ -599,6 +599,8 @@ fn type_last_layer(
             Ok(RetTypeOnly(text_type))
         }
         BinOp(RightBiasedRecordMerge, l, r) => {
+            use crate::phase::normalize::merge_maps;
+
             let l_type = l.get_type()?;
             let l_kind = l_type.get_type()?;
             let r_type = r.get_type()?;
@@ -614,7 +616,7 @@ fn type_last_layer(
             );
 
             // Extract the LHS record type
-            let mut kts_x = match l_type.to_value() {
+            let kts_x = match l_type.to_value() {
                 Value::RecordType(kts) => kts,
                 _ => return Err(mkerr(MustCombineRecord(l.clone()))),
             };
@@ -627,15 +629,13 @@ fn type_last_layer(
 
             // Union the two records, prefering
             // the values found in the RHS.
-            for (label, value) in kts_y {
-                kts_x.insert(label, value);
-            }
+            let kts = merge_maps(&kts_x, &kts_y, |_, r_t| r_t.clone());
 
             // Construct the final record type from the union
             Ok(RetTypeOnly(tck_record_type(
                 ctx,
-                kts_x.iter()
-                     .map(|(x, v)| Ok((x.clone(), v.to_type()))),
+                kts.iter()
+                   .map(|(x, v)| Ok((x.clone(), v.to_type()))),
             )?
             .into_type()))
         }
-- 
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